Thank you for you great answers.
Geoff: the temperature of the alternator with the engine running is higher, over 194°F at the shown point. And yes, I cleaned the engine bay a whole weekend, including the chrome fan, hehe. I bought a tornador gun, which is a great helpful tool for any type of cleaning.
Some more questions:
I marked one of the hoses on the above picture. Does anyone know where to get this hose with the original yellow stripe?
Also I'm looking for a throttle cable in the original green color (also marked).
And I'd like to repaint the valve covers, but not sure what's best practice to handle the unpainted parts (MG logo etc.).